The invention is in the field of optical reflectors and more particularly in the field of reflector lamps.
One general type of reflector lamp comprises a concave reflector having a parabolic contour with respect to a focal point, so as to reflect frontwardly light emitted by a light source located at the focal point. The cross section of the reflector usually is circular, the diameter thereof varying with the distance from the focal point. Additionally, a cone of light rays directly from the light source at the focal point pass, unreflected, through the front of the reflector, the angle of this cone of rays being determined and defined by the front rim of the reflector. The more widely divergent light rays of the cone of rays, i.e. the rays passing relatively nearer to the rim of the reflector, have such a large sideways component of direction so as to fall outside of the desired light pattern and therefore are wasted. The wasted, divergent light can be reduced, and the optical efficiency improved, by making the reflector deeper (longer) so that relatively more of the light is reflected in the desired direction and the cone of non-reflected light is narrowed. However, there are practical limitations on increasing the depth of the reflector, such as cost, weight and awkwardness of use. Also, with a given maximum diameter as the reflector is made deeper, the focal point moves closer to the rear surface, which complicates positioning of the light source and if the light source is a filament there is accelerated blackening of the nearby rear area of the reflector due to evaporation of the filament material (usually tungsten). This accelerated blackening can be alleviated by providing a concave recess at the rear portion of the reflector, which has the drawback of reducing optical efficiency.
Reflectors have been designed having combinations of parabolic and spherical shapes. For example, U.S. Pat. No. 2,629,046 shows a reflector having a parabolic front section, a spherical intermediate section, and a spherical rear section. U.S. Pat. No. 1,799,711 shows an automobile headlamp reflector having a parabolic front section, a spherical intermediate section, and a parabolic rear section, these sections being tilted with respect to each other so as to have different focal points. Other reflector shapes have been proposed, such as an ellipsoidal reflector lamp as is disclosed in U.S. Pat. No. 4,041,344.